Is Citrobacter koseri an AmpC (Ambler class C beta-lactamase) producing bacterium?

Citrobacter koseri and AmpC Production

Citrobacter koseri is not an AmpC-producing bacterium, unlike its relative Citrobacter freundii which naturally produces chromosomal AmpC β-lactamases.

Bacterial Classification and Resistance Mechanisms

Citrobacter species belong to the Enterobacteriaceae family, but they differ in their intrinsic resistance mechanisms:

• Citrobacter freundii: Naturally produces chromosomal AmpC β-lactamases (cAmpC) that can be inducible or constitutive 1
• Citrobacter koseri: Does not naturally produce chromosomal AmpC β-lactamases 1, 2

This distinction is clinically important because:

1. AmpC-producing organisms can appear susceptible to third-generation cephalosporins initially but develop resistance during therapy
2. Treatment failures may occur when using certain β-lactam antibiotics against AmpC producers

Resistance Patterns in Citrobacter koseri

While C. koseri lacks intrinsic AmpC production, it can acquire resistance through other mechanisms:

• Plasmid-mediated resistance: C. koseri can acquire plasmid-encoded β-lactamases including:

  • Extended-spectrum β-lactamases (ESBLs), particularly CTX-M-2 3
  • Carbapenemases such as OXA-48-like enzymes (e.g., OXA-181) 4

• Prevalence of acquired resistance: Studies have shown that ESBL-producing C. koseri can be regionally prevalent, with rates as high as 32.1% in some Japanese studies 3

Treatment Considerations

When treating infections caused by Citrobacter koseri:

1. Antibiotic options:

   • C. koseri is typically susceptible to third-generation cephalosporins (unlike AmpC producers)
   • Carbapenems are effective for serious infections 5
   • Cefepime is often effective against C. koseri infections 1

2. Monitoring for acquired resistance:

   • Susceptibility testing is essential as C. koseri can acquire plasmid-mediated resistance genes
   • For ESBL-producing strains, carbapenems are often the preferred treatment 6

Clinical Implications

The lack of intrinsic AmpC production in C. koseri has important clinical implications:

• Third-generation cephalosporins can be used effectively if the isolate is susceptible
• There is less concern about inducible resistance developing during therapy compared to C. freundii
• However, acquired resistance through plasmids remains a concern, particularly in healthcare settings

Understanding this distinction helps guide appropriate antibiotic selection and reduces the risk of treatment failure when managing C. koseri infections.